The present invention relates to a novel acrylic-functional organopolysiloxane and a method for the preparation thereof. More particularly, the invention relates to an organopolysiloxane having a plurality of (meth)acryloxy-substituted alkyl groups as the functional groups at the molecular chain ends and an efficient method for the synthetic preparation of such an acrylic-functional organopolysiloxane.
It is well known that a high molecular-weight organopolysiloxane can be converted into a silicone rubber elastomer having excellent heat and cold resistance, weatherability and electric properties by heating in the presence of an organic peroxide as a curing agent. It is also known that a high molecular-weight organopolysiloxane of a certain functional type can be cured into a rubbery elastomer by the irradiation with ultraviolet light in the presence of a photosensitizer.
The ultraviolet-curable organopolysiloxane compositions heretofore known include those in which the crosslinking reaction proceeds between a vinyl group-containing organopolysiloxane and a mercapto group-containing organopolysiloxane by the photochemically induced free-radical addition reaction (see, for example, Japanese Patent Publication No. 52-40334 and Japanese Patent Kokai No. 60-104158). The organopolysiloxane compositions of this type have a disadvantage that the application field thereof is very limited due to the unpleasant odor more or less emitted by the mercapto group-containing organopolysiloxane as is the case in any mercapto-containing organic compounds as well as the corrosiveness of the composition to the surface of a metallic article in contact therewith due to the mercapto groups.
Ultraviolet-curable organopolysiloxanes of another class include those comprising an organopolysiloxane having at least two acrylic groups in a molecule and a photosensitizer disclosed in, for example, Japanese Patent Publication No. 53-36515 and Japanese Patent Kokai No. 60-215009. Since these acrylic-functional organopolysiloxanes must have a linear molecular structure of a quite high molecular weight and the acrylic groups are bonded only to the terminals of the polymeric molecules in order to be able to give a rubbery elastomer by curing, it is a natural consequence that the number of the acrylic functional groups is relatively very small in the so large molecules so that the curability of the composition is usually low and the curing reaction sometimes does not proceed to completion, especially, in the surface layer due to the inhibiting effect of the atmospheric oxygen on the flee-radical reaction. This is the reason for the fact that utilization of the curing mechanism by the ultraviolet-induced addition reaction in the acrylic-functional groups is limited to certain resinous compositions in which a multiple number of the acrylic-functional groups can be introduced per molecule of the resinous organopolysiloxane having a branched molecular structure.
Japanese Patent Kokai No. 63-183911 discloses an ultraviolet-curable organopolysiloxane composition comprising an acrylic-terminated diorganopolysiloxane of a linear molecular structure and a photosensitizer, of which the acrylic functional groups are introduced into the terminals of the linear organopolysiloxane molecules by using an acryloxyalkyl-containing silanol compound. Although the organopolysiloxane composition of this type is capable of giving a rubbery elastomer by curing, such a composition is not suitable for industrialization because the process for the preparation of the acrylic-functional organopolysiloxane is very lengthy and complicated consequently resulting in an increase in the costs.